Bill Gates, 62 years old, is the second-richest person in the world. He's the former CEO of Microsoft and the cofounder of the Bill and Melinda Gates foundation.

Bill Gates
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Source: Bloomberg

Warren Buffett, 87, is the fourth-richest person in the world. He's a legendary investor and the CEO of Berkshire Hathaway.

warren buffett
Paul Morigi/Getty Images

Source: Bloomberg

Gates' and Buffett's friendship dates back to July 5, 1991. Gates' mother, Mary, had invited the late Meg Greenfield, a Washington Post editor, to her home. Greenfield was to bring her friend Buffett with her.

young bill gates 1992
AP Photo

Source: Fortune, Business Insider

Mary Gates pressed her son to join them — he didn't want to take the day off work. "What were he and I supposed to talk about, P/E ratios?" Gates later wrote. The only reason he decided to come was because Greenfield was also bringing the late Post publisher Katharine Graham and he was interested in the Post's history.

Katharine Graham
Katharine Graham.
AP

Source: Fortune, Business Insider

Buffett didn't particularly want to meet Gates, either. "While we're driving down there, I said, 'What the hell are we going to spend all day doing with these people? How long do we have to stay to be polite?" Buffett remembered years later.

warren buffett 1995
Mark Cardwell/Reuters

Source: Business InsiderThe Snowball

To the surprise of both men, they hit it off. Gates recalls being impressed by the questions Buffett asked him, like, "If you were building IBM from scratch, how would it look different?" And Gates told Buffett to buy stock in Intel and Microsoft. It was the beginning of a deep friendship and a mutual mentorship.

warren buffett bill gates
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Source: Fortune, Business Insider

Shortly after that first meeting, Gates asked Buffett for his favorite business book recommendation. Buffett lent Gates his copy of "Business Adventures" by John Brooks. Today, it's Gates' favorite business book, too — and he still has Buffett's copy.

bill gates business adventures
Ramin Talaie/Getty; Amazon

Source: The Wall Street Journal

Both credit their tremendous success to their laser focus. Gates has written that he's learned from Buffett how to manage his time by prioritizing certain people and tasks.

bill gates warren buffett talking
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Source: LinkedIn, LinkedIn

They've often joined forces for political and philanthropic causes. In 2010, Gates and Buffett, along with Gates' wife, Melinda, started The Giving Pledge. Billionaires who sign up commit to giving away more than half their wealth.

Bill Gates Melinda Warren Buffett
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Source: The Giving Pledge

In fact, Buffett contributes part of his fortune to the Bill and Melinda Gates Foundation every year. In 2017, he donated $2.42 billion of Berkshire Hathaway stock.

bill gates warren buffett laughing
Nati Harnik/AP Images

Source: CNBC

In 2014, Gates and Buffett, along with Sheldon Adelson, published a New York Times op-ed pushing for immigration reform.

How Bill Gates and Warren Buffett are changing the world like no other humans in history
Getty

Source: New York Times

The friends also have some odd pastimes. They participate in the newspaper-tossing challenge at the annual Berkshire Hathaway meeting.

bill gates warren buffett newspaper
REUTERS/Rick Wilking

Source: Business Insider

And they recently learned that they like trying out mattresses together.

bill gates warren buffett
The Gates Notes, LLC

Source: Business Insider

Gates has said Buffett's number is one of just two he has on speed dial at the Bill and Melinda Gates foundation.

warren buffett bill gates
Rick Wilking/Reuters

Source: GatesNotes

The friendship has made a big impact on both men. Gates wrote that "Warren has helped us [Bill and Melinda] do two things that are impossible to overdo in one lifetime: learn more and laugh more."

warren buffett bill gates ping pong
Rick Wilking/Reuters

Source: GatesNotes